1. Field of the Invention
A magnification-demagnification optical system and, more particularly, to such an optical system suitable for projecting a high-power-density beam of radiation manifesting a demagnified precise image of a given mask or reticle pattern on an ablation surface of a substrate, such as the surface of a coated semiconductor, for the purpose of ablating the ablation surface in accordance with the pattern image.
2. Description of the Prior Art
Both transmissive (lens) element and reflective (mirror) element systems for deriving a projected light image are known in the art. In the design of such systems, it is usually desired to reduce image aberrations to a minimum or, at least, to an acceptable level within complexity and cost constraints. It is not practical to attempt to solve the large number of non-linear ray-tracing equations of design variables that it usually takes to determine exact element positions for the design that introduces the minimum image aberrations. Therefore, in practice, simplifications are made that permit "starting points" for the element positions that are only somewhere in a general vicinity of the exact element positions to be determined. After which, known iterative computer software programs are employed for moving the element positions from their respective "starting points" toward their respective exact positions at which optical aberrations are reduced toward a minimum and a specified magnification or demagnification is obtained. Examples of such computer software programs are "Code V", available from Optical Research Associates, 550 North Rosemead Boulevard, Pasadena, Calif. 91107, and "Super Oslo", available from Sinclair Optics, 6780 Palmyra Road, Fairport, N.Y. 14450. In principle, the optimization of the optical design performed by such computer software programs could be performed analytically instead. However, because the analytical way of performing optical design is very tedious and time consuming, it is significantly less practical than optical design performed by computer software programs.
As known in the art, transmissive projection optics employing a system of one or more lenses is commonly used for most imaging purposes. A lens system has several design variables available (e.g., lens shape, lens power, lens separation and lens material) that can be used to control the many image aberrations that must be optimized for good performance. However, transmissive projection optics are not suitable for implementing the aforesaid surface ablation because absorption of heat by one or more of the lenses of such transmissive projection optics due to a high power-density beam of radiation passing therethrough is at least sufficient to degrade the image quality and may be sufficient to destroy a lens. By way of example, consider the case in which a beam of ultraviolet radiation from a high-power excimer laser (e.g., at least 100 watts) having a wavelength of 0.24840 micrometer (.mu.m), after passing through a mask defining a given pattern, is demagnified at its image plane by a factor of 5 in each of its two cross sectional dimensions. The result is that the demagnified beam in the image plane has a power density of 5.sup.2 or 25 times higher than the already high power density that the beam had in passing through the mask. The very high power density of the demagnified beam in the image plane is needed to effect the the aforesaid surface ablation. Other useful ultraviolet radiation wavelengths of a high-power excimer laser are 0.308 .mu.m and 0.196 .mu.m.
A mirror absorbs very little of the radiation energy incident thereon. Therefore, reflective projection optics employing only a system of mirrors would be suitable for implementing the aforesaid surface ablation. However, the physical presence of one mirror of a multi-mirror system tends to get in the way of the light path to another mirror of the system, and thereby obscures or vignettes parts of the light beam. This makes it particularly difficult to implement reflective projection optics employing a relatively large number of mirrors to transmit a large field of view without serious energy loss due to mirror blockage of the light path. Further, since a mirror has fewer degrees of design freedom than a lens, in order to obtain a sufficient number of design variables to overcome image aberrations, it takes a large number of mirrors or the use of aspheric surfaces to achieve the the same number of design variables that even a simple lens system possesses. Since the light blockage problem is compounded as the number of mirrors required to reduce image aberrations to a negligible level becomes greater, fewer mirrors having aspheric surfaces are normally employed in reflective projection optics to obtain sufficient design variables. Because aspheric surfaces are much more difficult and expensive to make than are spheric surfaces, a reflective projection system employing only spheric mirror surfaces that introduce only negligible optical aberrations would be most desirable for a purpose such as ablating a surface in accordance with a projected demagnified pattern image.